1. Technical Field
The present invention relates to a laser wavelength conversion apparatus, and more particularly, to a laser wavelength conversion apparatus which converts a wavelength of a laser beam to be applicable to precise machining.
2. Description of the Related Art
As a method for high-speed precise machining, ultraviolet laser machining is known in the art. In order to obtain an ultraviolet laser beam, a method of converting an infrared laser beam may be used.
In the method of converting an infrared laser beam into an ultraviolet laser beam, first, an infrared laser beam is subjected to partial wavelength conversion into a green laser beam through a second harmonic generator (SHG). Then, the green laser beam and the infrared laser beam are focused and combined through a sum frequency generator (SFG), thereby generating an ultraviolet laser beam. As the sum frequency generator, a non-linear optical material may be used. For example, lithium triborate (LBO) may be used.
To perform high-speed precise machining, an ultraviolet laser beam is required to have high repetition rate, high power, and high beam quality. In the case where the ultraviolet laser beam is generated by converting the wavelength of the infrared laser beam, the high repetition rate of the infrared laser beam causes decrease in pulse energy and peak power, and thus the size of the laser beam must be reduced by focusing the laser beam appropriately in order to increase wavelength conversion efficiency.
However, as the laser beam becomes smaller, chromatic aberration of a focusing lens becomes serious. When an infrared laser beam and a green laser beam are focused by an ultraviolet generating material, the infrared laser beam and the green laser beam are generally focused at different positions due to chromatic aberration of the focusing lens. In this case, interaction between the beams becomes weaker and causes deterioration in conversion efficiency for an ultraviolet beam. This phenomenon becomes more serious as the size of the focused beam size is reduced.
In order to solve such a problem, a method of using a concave mirror instead of a lens (see High efficiency generation of 355 nm radiation by extra-cavity frequency conversion: Optics Communications 283 (2010) 3497-3499) has been proposed in the related art. However, this technique has problems in that beam alignment is complicated and astigmatism of a spherical mirror occurs.